Field of the Application
The present invention relates to metallurgical practice, and more particularly to a tank furnace for hot metal coating. For example, the invention is readily adapted for application in the hot-coating of a steel wire or band with copper-base alloys.
As widely employed in the art of hot metal coating the following steps normally include: removing impurities and oxide films from the articles to be coated, submerging said articles in a molten coating metal and subsequently cooling them in the medium preventing the coating from oxidation.
The method described above is performed on any conventional apparatus for hot metal coating, which comprises a container for a molten metal, a means for cleaning articles to be coated before immersion, and a means for protecting the coating against oxidation.
It is now widely accepted practice to use an apparatus for hot-dip metal coating, which incorporates liquid devices intended to clean the articles being coated.
In addition to a container for molten coating metal, the prior-art apparatus comprises a container for a liquid chemical cleaner positioned in front thereof in the direction in which the articles are advanced (cf. Swedish Pat. No. 329068, Cl.48b (08). The apparatus of this type fails to ensure sufficient efficiency of the process because of the necessity to convey articles between the chamber and the container. Moreover, the apparatus is complicated in construction and requires much floor space.
The above disadvantages are not inherent in the apparatus in which a molten flux, having the same temperature as a molten coating metal, is used as a means for cleaning the articles to be coated.
U.S. Pat. No. 2,751,311 describes a tank furnace for metal coating, comprising a stationary body with a chamber containing a molten flux and a container filled with a coating metal. The chamber is fitted with electrodes for melting a flux and maintaining a predetermined temperature.
The furnace of the patent referred to above in provided with a hoisting mechanism which is mechanically linked with the container through a pusher bar pivoted thereto and interacting with a cam mounted on the rod of a fluid actuated cylinder. Thus, the container has two fixed positions in height relative to the furnace chamber. In both positions, upper and lower, the container is found inside the furnace chamber, with the level of molten metal in the container being lower than that of molten flux. The furnace is equipped with a mechanism adapted for advancing work pieces to the coating zone and provided with a cam manipulated by a piston and rod combination actuated by a fluid cylinder. In operation, the suspended articles advance under pusher bar action in the upper part of the furnace chamber above the container. The depth of immersion of the articles being coated is controlled by a means provided in the form of a screw pair driven by a hand wheel and kinematically connected to the hoisting mechanism.
At the initial moment of the furnace operation the container is brought in its lower position so as to enable the work pieces to be advanced thereabove by the feeding mechanism to the position for coating.
Next, the container is raised by the hoisting mechanism until the molten metal dips the work to the predetermined level and time. After the immersion is completed, the container is returned to its initial position.
To compensate for exhaustion of molten metal in the bath, the cam of the hoisting mechanism is carried forward relative to its pusher with the aid of the screw pair of the depth-of-immersion control device, whereas the container is raised to a desired level relative to the work piece being coated. However, notwithstanding high rate of the coating process, the above-described tank furnace sometimes fails to ensure uniformity of the coating chemical composition. The reason for this is a lowering level of molten flux above the bath surface of molten metal in the process of coating due to a higher rate of flux consumption over the consumption rate of molten metal. It has been experimentally found that the chemical composition of the coating based on the alloys composed of several metals depends upon said level value. For example, in hot brassing of a steel work piece, zinc losses in the coating over the losses of the coating metal depend upon level of the molten flux above the bath surface of molten brass.
Therefore, in the furnace of the above-described construction it is possible to maintain the body of molten flux at constant level above the molten metal bath surface by moving the container relative to the furnace chamber with the aid of the hoisting mechanism. This, however, causes the depth of immersion to deviate from a preset value. Using respective control means for readjusting the predetermined depth of immersion will again upset the level of molten flux above the molten metal both surface, since the depth-of-immersion control means is connected with the container through the hoisting mechanism.
In brass coating of continuously advanced flexible lengthy articles, such as wire or band, a drop in the level of the flux body results in the non-uniform chemical composition of the coating, adversely affecting its properties. Any deviation from a predetermined depth of immersion interrupts the time interval required for the immersion of the article being coated beneath the bath of the molten metal. This, in turn, fails to ensure a uniform thickness of the metal coating and impairs its corrosion resistance.